It's likely your wheel cylinders are frozen. Rusted and won't move. You could try pulling the drum off and going at it from the front. Ultimately, you'll want to either repalce or rebuild the wheel cylinders. If you have the money, an alternative is to install rear disc brakes. TSM in Castle Rock makes a nice kit. Complete it's around $500 including loaded GM calipers. Rebuilding the cylinders is a PITA, but probably the cheapest way to go. If you have one that's really bad you'll have to replace it. You probably won't know until you have it apart. When you're in there you might want to replace the brake shoes. That's why it's worth considering the disc conversion. Jeff will probably pipe in and say he thinks drums stop better. Discs aren't bothered by being wet like drums are. Once you've converted to discs it's less maintenance because there is no adjusting involved.